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complex mental states like deception to others. It could arise through relatively simple associative processes, by which animals learn to avoid individuals whose presence is associated with a negative experience. Such associations may underlie contingent cooperation in flycatchers, for example.

Indeed, mental state attribution may be irrelevant to contingent cooperation in animals. Schino and Aureli (2009) have argued that the focus on cognitive constraints in discussions of contingent cooperation confuses proximate and ultimate explanations for behavior. Altruistic behaviors may be favored by natural selection because of the subsequent benefits they confer, but what motivates animals to behave altruistically are the previous benefits they have received. In this view, the accumulation of multiple, cooperative exchanges over time causes animals to form partner-specific emotional bonds that prompt future altruistic behavior. Thus, reciprocity may be maintained by a kind of partner-specific “emotional bookkeeping” (Schino and Aureli, 2009) that permits long-term tracking of multiple partners and facilitates cooperation in different behavioral currencies. The resulting bonds that develop between preferred partners may motivate future positive interactions without the need for explicit tabulation of favors given and returned, or calculations of anticipated benefits (Aureli and Schaffner, 2002). For unrelated females who interact at low rates, a single grooming bout may temporarily elevate a female’s positive emotions toward her partner sufficiently above baseline to influence her immediate interactions with her. In contrast, grooming and support among females with close bonds (who are also usually kin) should be less subject to immediate contingencies and less influenced by single interactions. Many of these proximate mechanisms may also motivate social interactions in humans. It seems unlikely, for example, that the formation of close bonds among humans is driven by expectations that such bonds will enhance health and longevity.

Finally, it is important to emphasize that, although the absence of punishment in animals may derive partly from cognitive constraints, a strict accounting of services given and received is likely maladaptive in groups in which individuals establish close bonds and interact regularly with familiar partners in a variety of contexts. In fact, although the cognitive constraints that supposedly limit contingent cooperation in animals is often contrasted with humans’ sensitivity to inequitable exchanges, human friendships are rarely contingency-based. Numerous studies have shown that people seldom keep tabs of costs and benefits in interactions with regular partners (Silk, 2003). Although people become resentful and dissatisfied when exchanges within a friendship are consistently unbalanced, tallying of favors given and received are typically reserved for infrequent associates. There is even some question about the extent

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